(Invited) Electro-Chemo-Mechanics at Solid Electrolyte-Cathode Interfaces

Abstract

Strain is present at interfaces of solid electrolytes with cathode and anode in solid state batteries due to interfacial reactions and due to volumetric expansion and contraction of the electrodes during battery charge and discharge cycles. In this talk, first, I will present our findings on how a solid electrolyte-cathode interface degrades chemically and electrochemically, and its implications on the interface mechanical degradation. In particular, we have studied the garnet type Li7La3Zr2O12 (LLZO) solid electrolyte with thin film LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode as a model system to enable interface sensitivity. X-ray absorption spectroscopy, X-ray diffraction and Gibbs free energy analysis have enabled us to identify the chemical and electrochemical degradation pathways, and their implication on the mechanical instability at the interface. Second, is the reverse effect of such mechanical evolution, in particular the effect of elastic strain at the interface on Li-ion diffusion in the solid electrolyte. For this question, we have taken a model solid electrolyte, β-Li3PS4, and have performed Ab Initio Molecular Dynamics (AIMD) simulations to resolve the resulting changes in Li site stability and Li ion conductivity under different strain tensors. We have found that the strain tensors which compress the c-axis (+2% ab, –2% ac, –2% bc, –2% isotropic) increase the Li-ion diffusivity, and the strain tensors which stretch the c-axis (–2% ab, +2% ac, +2% bc, +2% isotropic) reduce it. The c-axis compression increases disorder in the lattice and promote jumps along all migration pathways. The results show that elastic strain can promote Li disorder and superionic conductivity, with an effect on Li-ion diffusion comparable to that of chemical doping, and is important to consider for accurate understanding and prediction of solid-state Li-ion battery interface properties.